Reactive compensators, often referred to as Static VAR compensators (SVC), control voltage and provide reactive power in AC power transmission systems. Practical applications of SVCs include maintaining voltage at or near a constant level under slowly varying conditions in response to load changes, correcting voltage changes caused by unexpected events (e.g., load rejections, generator and line outages, etc.), and reducing voltage flicker caused by rapidly fluctuating system loads. SVCs may also be used to improve power system stability, power factor, and current phase imbalance. Controlled switching of reactive elements in the SVC regulates the amount of capacitive/inductive loading applied to its AC transmission lines thereby modifying the reactive power delivered by the power generation system.
An exemplary SVC 10 is illustrated in FIG. 1 and includes plural thyristor-switched capacitors (TSCs). AC filters and thyristor-controlled reactors (TCR) are examples of other reactive compensators. FIG. 1 is a conventional one line power generation diagram for representing a single phase of a typical three phase AC power supply network. Five thyristor-switched capacitors (TSC1-TSC5) are shunt-connected across one phase of the AC power supply network to the transmission line 20 through a conventional reactive transformer 22. Of course, any number of TSCs, TCRs, etc. could be included in SVC 10. Each TSC typically includes in series a capacitor 32, a surge inductor 34, and a thyristor valve 36. Each thyristor valve 36 includes anti-parallel thyristors which are controlled by externally generated firing signals (not shown). V.sub.T is the single phase SVC voltage on transmission line 20. I.sub.SVC is the current flowing from the SVC 10 to the transmission line 20.
FIG. 2 shows a block diagram of a conventional, closed loop, SVC voltage regulator used to generate thyristor gate pulses to activate and deactivate the thyristors 36 to generate the SVC reactive current I.sub.SVC at appropriate times. Using conventional voltage measuring devices, i.e. volt meters (not shown), V.sub.T is measured for each of the three phases A, B and C of the three phases in the AC power system and averaged with the other phase voltages in a conventional voltage averaging unit 40. The average SVC voltage is then filtered in conventional ripple filter 42 to remove harmonic AC ripple, in particular at the six harmonic, and generate measured SVC voltage Vm. This feedback SVC voltage Vm is then compared with a desired reference voltage Vref in summer 44 which generates a difference or error signal. The goal of the voltage regulator is, of course, to cause the SVC voltage Vm to move toward and be maintained by the desired voltage Vref.
The error is initially processed in the control loop by conventional deadband circuit 46 which desensitizes the control system response to relatively small changes in Vm. Unless the magnitude of the error signal is greater than some deadband threshold amount, no signal is output from deadband circuit 46 and no control action occurs. As will be illustrated below, desensitization is necessary to stabilize the system response and prevent unnecessary, oscillatory firing of the SVC thyristors. An integrator K/s 48, (where constant K has units of measurement MVARS/sec/unit voltage error), integrates the output signal from the deadband circuit 46 and generates a commanded susceptance Bo. The commanded susceptance Bo is processed in a conventional SVC firing control unit 50 which, based on Bo, determines the appropriate thyristor gate pulses for turning the TSC thyristors "on" so that the SVC provides reactive current ISVC to transmission line 20. This added reactive current causes the measured transmission line voltage at the SVC bus to move back toward the desired voltage Vref.
The SVC firing control unit 50 also may incorporate a hysteresis compensation factor B.sub.HYST which compensates for the practical hysteresis effects that delay the control system response to the newly commanded susceptance Bo. In effect, this hysteresis factor sets the commanded susceptance Bo at a constant value for a sufficient period of time to ensure that the system has had an opportunity to respond to the commanded direction before the current Bo command is removed or otherwise modified.
Instead or in combination with the deadband circuit 46, a slope setting unit 52 may be included in the secondary feedback loop feeding the commanded susceptance Bo back to summer 44 after multiplying it by a "droop" factor Dr. In essence, this secondary feedback input desensitizes the responsiveness of the primary feedback loop depending upon how much the susceptance Bo has changed. As a result, a large ordered susceptance (representative of a change in system demands) tends to decrease the difference or error output by summer 44.
Thus, by controlling either or both the deadband circuit 46 or the slope setting unit 52, the sensitivity of the conventional closed loop controller may be increased. To illustrate the need for desensitizing the control response of a conventional SVC voltage regulator, reference is made to FIGS. 3(a) and 3(b) which shows an exemplary high sensitivity response, i.e. without deadband or droop. A reference SVC voltage Vref (the dotted line) and the actual measured SVC voltage Vm (the solid trace) are plotted in per unit (pu) voltage against time in seconds. Although the goal of the SVC voltage regulator is to maintain Vm as close to Vref as possible, this is difficult to do in practice as reactive power (e.g. susceptance) is added to the power supply system. FIG. 3(b) shows susceptance B being supplied to the power system increasing in approximately stepwise fashion from 0.0 to 1.0 pu mhos during the period 0.00 seconds to 1.75 seconds. The ordered value of susceptance Bo is shown as the dotted trace and the actual value of SVC susceptance Ba is shown as a solid trace.
Each step increase or new command of Bo, i.e. at 0.05 seconds, 0.40 seconds, 0.95 seconds, and 1.35 seconds, generates an oscillating response of considerable duration referred to as "chatter" in SVC voltage Vm and susceptance. Chatter is caused by the system continually responding too far in one direction and then over correcting in the opposite direction thereby generating extended periods of overcompensating oscillatory corrections.
Such local oscillations can be reduced to some extent using deadband unit 46 and slope setting unit 52 in a conventional SVC voltage regulator as shown in FIGS. 3(c)-3(f). FIGS. 3(c) and 3(d) illustrate the system response when deadband unit 46 is activated. The addition of deadband eliminates the chatter observed in the high sensitivity example shown in FIGS. 3(a)-3(b). However, Ba is offset from Bo, and Vm is offset from Vref for considerable periods after each commanded change in susceptance Bo. FIGS. 3(e) and 3(f) also show reduced chatter when the slope setting unit 52 is activated. Unfortunately, the offsets between the measured and ordered values of both voltage and susceptance are even larger than those associated with deadband.
For additional discussion of conventional SVC dosed loop control systems with gain supervision, desensitization, and hysteresis adjustment to ensure stability of local oscillations, see "Digital Control of SVC Plants," by Wild et al, Canadian Electric Association, March, 1989, Toronto, and "Pre-Commissioning and Preliminary Control System Test Results for the Kemps Creek Static Bar Compensators, Australia," Henner et al, Canadian Electric Association meeting, March, 1989, Montreal.
Conventional closed loop control provides mechanisms for stabilizing local oscillations in SVC voltage regulators. Unfortunately, stability is achieved at the cost of accuracy and speed in response to changes in reactive power demands. Accordingly, what is needed is an improved SVC voltage regulator that regulates the voltage at the SVC bus to achieve sensitive and stable response to changes in the reactive power needs of the power supply network.
The present invention provides such a voltage regulator for regulating the voltage of a static VAR compensator (SVC) connected to a power supply network. An SVC status detecting means detects the conduction status of the SVC thyristors and generates the measured SVC susceptance in accordance with that conduction status. A calculator calculates a representative voltage that is the Thevenin equivalent of the power supply network based on the measured SVC susceptance, measured SVC voltage, and an estimate of the power supply network's equivalent reactance. A predictor then predicts an ordered susceptance that should be provided by the SVC based on the representative voltage to maintain the SVC voltage at a desired level. A firing control unit receives the predicted susceptance value and calculates appropriate drive signals for controlling thyristors in the SVC to thereby deliver sufficient reactive current to the power network in accordance with the ordered susceptance.
Another embodiment of the present invention includes a voltage regulator for a static VAR compensator (SVC) switchably connected to a power supply network at a bus. The voltage regulator includes means for measuring a susceptance delivered by the SVC to the power supply network; means for measuring an SVC voltage at the bus; means for modelling the power supply network using an equivalent network including an equivalent voltage and an equivalent reactance, the equivalent voltage being determined based on the estimate of the equivalent reactance, the measured SVC susceptance, and the measured SVC bus voltage; means for predicting a desired SVC susceptance based on the equivalent voltage and a reference SVC bus voltage; and means for controlling switching of the SVC based on the predicted SVC susceptance.
A method according to the present invention includes the steps of measuring the SVC voltage; determining an equivalent of the power supply network voltage based on the reactive power delivered to the network by the SVC, the measured SVC bus voltage, and an estimate of the power supply network's reactance; predicting a desired reactance to be delivered by the SVC based on the determined power supply network voltage; generating SVC control signals based on the predicted reactance; activating the SVC in accordance with the control signal; detecting the activation status of the SVC; and generating a measured reactance in accordance with the detected SVC activation status. The measured reactance is representative of the reacted power delivered to the power supply network.
In another embodiment of the present invention, after the equivalent reactance is initially estimated, it is thereafter modified automatically to track and compensate for large disturbances in the power transmission network. This modification is determined based on changes in the calculated representative voltage and in the measured SVC susceptance via a correlation procedure. These correlations are accumulated and used to adjust the estimate of Thevenin reactance until the correlated changes in the representative voltage (due to changes in the measured SVC susceptance) are reduced to zero. The means for tracking changes in the representative voltage and measured SVC susceptance and the means for automatically modifying the estimate of the equivalent reactance in response to those changes permit the present invention to adapt to severe power system contingency conditions.
This Thevenin-based approach to modelling the power supply network from the perspective of the SVC permits predictive SVC voltage regulation that is sensitive, accurate, and stable. These and other objects, features, and advantages of the present invention will become aware to those skilled in the art from the following description and drawings in which: